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Do you have a harder time losing weight than your workout buddy who eats more than you? Do you struggle with your weight despite eating right, and wonder if your genetics are to blame? Is obesity a disease dictated by genes?

Obesity affects 78 million adults and 30 million children in the United States, and is defined as having a height-to-weight ratio score, a.k.a. body mass index (BMI), of 30 or above. For example, a person who is 5’4″ tall and weighs 175 pounds has a BMI of 30. This is the metric used to classify obesity as a disease by the American Medical Association, American Heart Association, American College of Cardiology, The Obesity Society, the Food and Drug Administration — even the IRS — and most of the doctors and dietitians interviewed for this story.

Is Obesity a Disease?

The National Institutes of Health categorizes diseases as either chronic or infectious, and says that chronic diseases are long-term and affect people of all ages and ethnic backgrounds. According to the NIH’s website, “Many chronic diseases have genetic components, which raise disease risk in certain people… the environment can also contribute to risk, and so can lifestyle choices, including your diet, physical activity, and whether or not you smoke.”

By definition, obesity can be considered a disease because it impairs normal function. “When people gain excess weight, there are abnormal changes that occur in the body, including abnormal signaling from fat cells, the endocrine system, and other organ systems, and dysregulation of appetite and energy balance,” says Adrienne Youdim, M.D., director of the Center for Weight Loss and Nutrition in Beverly Hills, California, and associate professor of medicine at UCLA David Geffen School of Medicine.

Excess weight is the most common sign of obesity, and it can affect every organ system in the body, according to Youdim. Obesity may contribute to cardiovascular disease, type 2 diabetes, and osteoarthritis. Caroline Apovian, M.D., director of the Center for Nutrition and Weight Management at Boston Medical Center, also considers how obesity reduces mental health and overall quality of life.

But, do all experts agree that obesity is a disease?

There is some controversy. On the one hand, declaring obesity a disease legitimizes it for treatment (and insurance coverage), which transfers more of the ownership for addressing it to physicians, and reduces the burden and stigma for the individual. All positive things.

On the other hand, some think BMI is not the best measure of obesity. Michael Coords, M.D., chief medical officer for MST Consulting, a medical education services company, admits, “It may do a decent job for the average person,” but he says it fails because it doesn’t take muscle mass into account.

A simple measure Coords thinks is more useful: belly fat, or abdominal obesity, which is a waist over 35 inches for women, and over 40 inches in men. “Ultimately, the best way to classify obesity would be to look at body fat percentage.”

Coords adds that many of his physician colleagues consider obesity more of a condition than a disease, but, “if classifying it as a disease allows patients to receive better access to appropriate care, then I can agree with that position.”

Is Obesity Caused by Nature or Nurture?

The rapid increase in obesity rates can’t be due to genetics alone, according to Katie Anderson, Ph.D., vice president of science for AthGene, a company that provides consumers with access to their DNA information. She points to the rapid rise in global obesity rates in the last 100 years. Today, nearly 40 percent of the world’s adults are overweight.

In the United States, it’s higher: two out of three adults are overweight. In just the past three to four decades, global rates of obesity have more than doubled. “This time span is simply too short to be explained by changes in genetics, which take hundreds to thousands of years to change significantly in a population.”

Instead, genes (nature) and the environment (nurture) intersect to contribute to obesity. The term for this interplay is “epigenetics.” Coords explains: “Epigenetics is basically an on/off switch. DNA provides all of our potential options but it doesn’t mean your body is using all of them at once. You can silence certain genes or express others without actually changing the DNA you have.”

Obesity is caused by a complex interaction between genetics and the environment, confirms Apovian. She provides this hypothetical example: If two children eat a high-fat, high-sugar diet for a month and child No. 1 is genetically susceptible to obesity and child No. 2 is not, their bodies will respond differently. Child No. 1 may develop pre-diabetic symptoms in a few weeks, her body weight set point (the number on the scale the body wants to return to) will change, along with a metabolism adjusted to keep her at a higher weight. Her hunger hormones would adapt to be consistently high, while satiety hormones would drop. Complex signaling from the brain would be altered as well, causing weight gain. In contrast, the second child would only experience these consequences if the high-fat, high-sugar diet is kept up for much longer than one month.

Child No. 1’s genes — the blueprint inherited from her parents that encodes everything from physical attributes to her likelihood to develop disease — favor weight gain. And it turns out, minute variations in DNA can have this effect. “Most of our genetic code is the same, with only very small changes,” says Anderson. “Quite often, the change is just one nucleotide that is mutated, and while this change is tiny, the effect it can have is huge.”

For example: a variation on the “FTO” gene.

What Is the FTO Gene?

The fat mass and obesity-associated (FTO) gene is the first and most studied gene in connection to obesity. One study published in Nature Genetics concluded that FTO contributes to common, or polygenic, human obesity. Polygenic obesity is obesity caused by multiple factors, of which, genetics is but one part.

Higher levels of a specific variation on the FTO gene are linked to overeating (especially fatty foods), always feeling hungry, and difficulty feeling full — and this was seen in both kids and adults across African, Asian, European, Pima Indian, South American, and South Asian ethnicities.

While this doesn’t sound good, on it’s own, it can only predict 0.34 percent of obesity; and actually, FTO is essential for normal development of the central nervous and cardiovascular systems in humans. The FTO variation most studied as contributing to obesity is not strong enough of a link to predict obesity; however, there is still an association between FTO and multi-cause obesity. But the FTO variation alone isn’t what’s causing obesity.

In fact, our lifestyle choices, combined with our genetic makeup, can make some of us more likely to become obese than others. There is growing evidence that our eating habits interact with genes to influence our chances for becoming obese. For example, authors of a study published in Current Genomics noted that a high-fat diet can amplify the effect of the FTO genotype on obesity risk.

Anderson explains that this FTO gene variant helped our ancestors store fat and survive food scarcity. Over a long period of time, natural selection favored this gene variant, as those without this new mutation would have been less likely to survive periods of starvation, and pass on their genes. Since it confers such a strong advantage in times of food scarcity, Anderson hypothesizes that is why this particular gene variant became very common in the human gene pool.

“While this mutation in FTO served our ancestors well,” says Anderson, “when we fast-forward to modern times, the ability to convert food to body fat stores is in many cases no longer an advantage. Obesity is now a larger problem than starvation.”

Tell us something we don’t know.

Do Genes Dictate Future Health?

Thankfully, genes are not destiny when it comes to obesity. To begin with, our best estimates, which are preliminary, suggest that only up to 10 percent of obesity can be explained by genetics. Reisdorf notes that obesity is in a category of disease that may have a genetic predisposition, but lifestyle and environment determine if the disease will actually develop or not.

“It might be better to say that genes influence future health, rather than predict,” advises Apovian.

What Can You Do if Your Genes “Want” You to Gain Weight?

Sure, a genetic predisposition to gaining weight means having to work harder at a healthy lifestyle, but think about it as if you had a family history of heart disease. Apovian says, “As with any disease, knowing that your risk is higher, it is important to guard your health closely regarding triggers for obesity. ”

It’s possible to impact genetics with healthy lifestyle habits such as filling half your plate with vegetables at every meal, getting enough lean protein, enjoying healthy fats such as olive oil or avocados, and being physically active every day.